As a prior art, a fuel-saving system that effectively promotes fuel-saving driving using road gradient information and a control method thereof are disclosed (See Patent Document 1, for example).
In the prior art (Patent Document 1), improvement in fuel efficiency is promoted by using acceleration energy on a descending slope or in more detail by performing speed-reduction control before the descending slope.
In the prior art (Patent Document 1), an advice position for fuel-cut before the descending slope (a position where an advice is given to a driver) or a control position for fuel-cut in an automatic fuel-saving driving system (a position where fuel is cut by automatic control) is determined by a vehicle speed (a car speed), a gradient before the descending slope and the gradient of the descending slope. And the gradient before the descending slope and the gradient of the descending slope are determined by a three-dimensional map.
Thus, in order to reduce the speed to a targeted vehicle speed at start of the descending slope, the gradient before the descending slope and the gradient of the descending slope need to be mapped precisely, and the advice position for fuel-cut or the control position for fuel-cut in the automatic fuel-saving driving system also need to be mapped precisely.
However, even if the map is to be improved precisely, the gradient at an actual traveling position is not necessarily constant.
Also, in the case of a cargo vehicle, its mass is greatly varied whether the vehicle is empty or loaded, and thus, deceleration is largely different at the fuel-cut between the empty vehicle and the loaded vehicle, and it has been difficult to reduce the speed to a requested vehicle speed (target vehicle speed) at start of the descending slope both in the empty vehicle and the loaded vehicle.
Here, in the case of the automatic fuel-saving driving mode, at a requested vehicle speed or less, control is performed such that fuel is automatically injected so that the vehicle speed is not reduced excessively. Because of such control, fuel might be injected immediately before entering the descending slope, and in that case, a vehicle behavior becomes jerky, and a driver would have a sense of discomfort, which is a problem. At the same time, there is also a problem of a bad influence on the fuel efficiency.
FIG. 5 shows a traveling distance, a vehicle speed and a fuel injection amount in the control at a point to change to descending during the automatic fuel-saving driving according to the prior art.
In FIG. 5, reference character P1 denotes a control start position, reference character P2 for a peak point changing from ascending to descending (descending-slope starting point), reference character Vd for a requested vehicle speed, reference character Va for an actual vehicle speed, reference character Vt for a target vehicle speed when entering the descending slope, and reference character q for a fuel injection amount. Here, a distance from the control starting point to a descending starting point is 300 m (constant), for example.
In FIG. 5, reference character by indicates a difference between the actual vehicle speed Va and the target vehicle speed Vd.
FIG. 5 shows a case in which the gradient on the ascending side is steeper than that in data stored in a database or a vehicle mass is small.
In the case shown in FIG. 5, the speed is lost before reaching the descending-slope starting point P2, and the vehicle speed Va becomes lower than the target vehicle speed Vt at entering the descending slope. Thus, the fuel injection amount q is temporarily injected. By means of this temporary fuel injection q, the vehicle speed Va rapidly increases and becomes higher than the target vehicle speed Vt at entering the descending slope, but since fuel is cut immediately after the fuel injection amount q is temporarily injected, the vehicle speed Va is decreased again.
As a result, a driving feeling before and after the descending-slope starting point P2 becomes jerky, and fuel efficiency is deteriorated by the temporary fuel injection.
FIG. 6 shows the traveling distance, the vehicle speed, and the fuel injection amount in control at a point of time changing from ascending to descending in the automatic fuel-saving driving of the prior art as well as shown in FIG. 5. However, FIG. 6 shows a case in which the gradient on the actual ascending side is gentler than that in the data stored in the database or the vehicle mass is larger.
In the case shown in FIG. 6, the actual vehicle speed Va does not fully slow down but the actual vehicle speed Va at the descending-slope starting point P2 exceeds the target vehicle speed Vt at entering the descending slope and a frequency of operating an auxiliary brake in the middle of the slope is increased, which deteriorates the fuel efficiency.
FIG. 7 shows a case in which in order to solve the problem in FIG. 5, the control start position P1 is brought close to the descending-slope starting point P2 and the distance from the control starting point to the descending starting point is set short.
In FIG. 7, it is not necessary to temporarily inject the fuel injection amount q before reaching the descending-slope starting point P2 as in the case shown in FIG. 5. However, similarly to the case shown in FIG. 6, the actual vehicle speed Va at the descending-slope starting point P2 exceeds the target vehicle speed Vt at entering the descending slope, and the frequency of operating the auxiliary brake in the middle of the descending slope is increased and thus, the problem of deterioration in fuel efficiency still remains.    Patent Document 1: JPA (Non-examined publication) No. 2007-156704